An attempt to evaluate the effect of Delta-9 tetrahydrocannabinol (THC) on the sensory and affective insensitivity to pain of iv morphine and to describe the adverse effects of individual and combination drug therapies. Clinical studies suggest the hypothesis that THC can make more effective the analgesic effect in humans. The objective of this study is to investigate this hypothesis under experimental conditions which will allow the detection of effectiveness if, indeed, it occurs. This will be done by comparing the insensitivity to pain produced by a combination of oral THC and intravenous morphine with the effects produced by morphine alone. If the hypothesis holds true in an experimental setting, the clinical implications may be twofold. First, the addition of THC may achieve pain relief in persons with chronic pain not relieved by narcotic analgesics alone or in combination with other conventional analgesics. Second, for some patients with adequate pain relief with narcotic analgesics alone or in combination with other agents might favorably alter the side effects profile of the analgesic regimen with equivalent, acceptable analgesia. THC is one of the active components of marijuana and can cause a high. It is currently prescribed as a medication to control nausea and vomiting induced by cancer chemotherapy and to stimulate appetite in people with AIDS. Recent laboratory studies indicate that low doses of THC may increase the pain-relieving effects of morphine. Subjects of this study may have no serious illnesses, must not be pregnant, must not have injested and opioid or cannabinoid within 30 days prior to enrollment, must have no allergies to opioids , cannabinoids, or placebos, and they must be able to arrange for transportation because of the effects the drugs may have. The subjects will have five visits to the clinic over a five week period. The first visit is a two hour training period, the other four will be test visits of six hours each. At the first visit, a medical history will be taken and a physical exam will be done. A urine test and a pregnancy test will be done. A heat probe will be applied to an arm to cause experimentally induced pain and the pain will be rated. At each of the next four visits, the following will take place: Another pregnancy test will be done. A small catheter will be inserted into a vein. Blood pressure, pulse, and breathing will be regularly measured. The drug or a placebo will be given and morphine will be given 90 minutes later. Sensitivity and tolerance to pain will be tested with the heat probe. There will be numerous heat probe exposures, each of only a few seconds. The subject will rate the unpleasantness and intensity of the pain. Then a questionnaire will be filled out concerning any physical or mental sensations that are experienced and considered to be due to the medication.